- How does Envirohemp go about replicating the carbonization and activation processes using pilot equipment, and how have these replications guided the adjustment of parameters to an industrial-like manufacturing level?
Activated Carbons, ACs, are a wide family of materials typically derived from the simultaneous or sequential carbonisation and gasification of biomass at high temperature. The pool of methodologies for AC production developed in the lab over decades is vast and for the most part remains untapped at industrial level. As a result, commercial ACs represent just a very small fraction of the different AC materials that are known to researchers. The main reason for this lack of industrial uptake is linked to scale-up and resource efficiency challenges. The industrial production of ACs currently relies on more or less simple industrial processes, based on the use of rotary furnaces and the insertion of water vapor as the main gasification agent. This approach offers very restricted possibilities in terms of engineering of the final product and leads to unremarkable materials, not matching the expectations of high-end applications such as the gas storage sector. The resulting AC materials are thus characterized by a modest porosity with values of surface area per gram of material that are at best 40% of their theoretical ceiling.
However, scaling-up the fancier, highly-activated, AC versions requires dealing with multiple engineering and practical problems. The so-called chemical activation route is among the most challenging ones as they involve harmful chemicals such as strong acids or alkali. Now imagine that you want to scale-up a process at a temperature so high that it makes most refractory steels become labile. Add to this a harsh environment generated by corrosive chemicals such as molten salts and eventually metal vapors produced as a by-product of the gasification reactions. And to top that off, try to accommodate to complex changes in the behavior of the matter including liquefaction, swelling to several times its original volume and a final solidification.
The major foreground generated by Envirohemp to replicate complex activation methodologies at pilot scale relies on two main developments: i) the design and validation of a simple, robust and scalable furnace cladding that can withstand the working conditions without significant contamination of the final products and ii) the fine-tuning of the operational conditions to cope with and, eventually, take advantage of unforeseen effects at lab-scale (e.g. secondary activation reactions produced by unvented exhaust gasses).
- What challenges have you encountered when scaling the protocols for the best candidate AC materials, and which mitigation strategies have been successful in reducing the discrepancies in yield and performance it experienced during the scale-up process?
In general, it has been consistently observed that the replication of lab-scale conditions in the pilot is subjected to the rise of new and unforeseen effects that make it difficult to transfer the protocol to the kg and multi-kg scale. This does not mean that the properties of the ACs produced at the pilot are insufficient. However, it is not straightforward under the pilot conditions which set of conditions will lead to a certain combination of AC characteristics.
The main mitigation action has been to produce a wide catalog of samples at the kg scale to generate a pilot-specific know-how on how the different activation conditions apply to the different available precursors. This catalog currently extends beyond 100 samples, generated from 12 different precursors. As a summary of the work performed so far it can be concluded that yields at pilot scale are typically good and require no mitigation actions, however, matching the whole spectrum of physicochemical properties has been slightly more difficult. In general terms, high values of porosity and surface area can be achieved at the kg scale, however, unusually high surface areas (>> 3000 m2/g) achieved in the lab were not yet accomplished by the pilot. More creative activation approaches will be attempted in the following months to try to overcome this limitation. On the other hand, the pilot activation has been extremely successful at providing unconventional micropore-mesopore hybridisation in some of the new AC materials produced, throughout the whole range of surface areas.
- As it scales the MOF materials and MOF composites, how does CSIR and Envirohemp guarantee the similarity of the physicochemical properties and performance the materials exhibit to lab-scale materials, and what is the role of the carbon materials that UoN has developed in favoring the hydrogen storage performance of the composites?
To ensure that MOFs materials and MOF composites maintain similar physicochemical properties and performance as their lab scale counterparts during scaling up from the lab, the CSIR employs adequate strategies.
During MOFs synthesis in the lab, process optimisation is well executed and a standard optimised procedure is recorded in the lab books. Conditions of temperature, reaction time, reaction ratios, and pressure in some cases are optimized. At each stage of lab scale synthesis, samples are characterized using appropriate techniques to ensure product quality and consistency. This step is key for validating that the properties of the MOF materials meet the desired specifications. The procedure recorded in the lab book serves as the basis for scaling up the process. Further optimisation is done when materials are now produced at scale to address any challenges that arise. Rigorous quality control is maintained throughout to ensure that the scaled up materials match the quality of lab scale products. In some cases, process conditions may be adjusted, while still maintaining product quality.
The role of the carbon materials developed by the University of Nottingham is to enhance properties of MOF composites. The carbon materials possess high surface areas and porosities, which can complement MOFs properties by providing additional sites for hydrogen storage. Additionally, the carbon materials are also expected to enhance the thermal conductivity of MOF composites, a property that is essential for hydrogen storage in cylinders for vehicular applications.
- Could you provide more details about the methods employed by Envirohemp and CSIR to densify carbon materials and multi-kg scale MOF and ACs, including the granulation and binder selection procedures, and how these methods affect the final prototypes’ mechanical stability and functionality in hydrogen storage applications?
This is part of the work that we are starting to perform at this point in the project. According to our past experience with ACs, extrusion is the preferred option to produce commercial AC pellets. In the case of highly-activated ACs there is an important knowledge gap both at pilot and industrial level. Some lab-scale studies suggest that part of the porosity might collapse and either disappear or lead to a narrower porosity under the application of high-pressures during the conformation of pellets. However, those conclusions were mainly gathered from die-cast pelletisation of lab-scale ACs. Envirohemp will therefore provide new and unprecedented evidence at the pilot scale on how highly-activated ACs behave under extrusion conditions.
At CSIR, we have successfully shaped MOFs and MOFs composites through granulation. The secret is to make sure that you use the best binder that does not compromise the MOFs surface area and porosity. The binder must also be cost effective to ensure that shaping the MOFs does not increase the material costs. Additionally, we plan to shape MOFs by pelletisation and compare this method with granulation.
